Cesium-lithium-borate crystal and its application to frequency conversion of laser light

ABSTRACT

The present invention provides a cesium-lithium-borate crystal, which can be used as a high-performance wavelength converting crystal, having a chemical composition expressed as CsLiB6O10, and substituted cesium-lithium-borate crystals expressed by the following formula:  
     Cs 1−x Li 1−y M x+y B 6 O 10    
     or  
     Cs 2(1−z) Li 2 L z B 12 O 20    
     (where, M is an alkali metal element, and L is an alkali earth metal element); a method for manufacturing same by heating and melting; and an optical apparatus using such crystals.

FIELD OF THE INVENTION

[0001] The present invention relates to a cesium-lithium-borate crystaland crystals with substituted chemical compositions thereof. Moreparticularly, the present invention relates to a cesium-lithium-boratecrystal and crystals with substituted chemical compositions thereofwhich are used as frequency converting nonlinear optical crystals inlaser oscillators and optical parametric oscillators used in blue andultraviolet lithography, laser microprocessing, scientific andindustrial measurement and laser nuclear fusion, a method formanufacturing same, and an optical apparatus using same.

PRIOR ART AND PROBLEMS

[0002] Laser oscillators used in ultraviolet lithography, lasermicroprocessing, scientific and industrial measurement and laser nuclearfusion must generate stable blue and ultraviolet rays efficiently. Oneof the methods to achieve the above object which is attracting thegeneral attention to day is the method to efficiently obtain blue andultra-violet rays by frequency conversion of a light source usingnonlinear optical crystals.

[0003] A pulse YAG laser oscillator, a type of laser oscillators, forexample, uses nonlinear optical crystals to convert frequency of a lightsource to generate the third (wavelength: 355 nm) or the fourth(wavelength: 266 nm) harmonics of the pulse YAG laser.

[0004] Many contrivances on the frequency converting non-linear opticalcrystals which are indispensable for generating ultraviolet rays havebeen announced. For example, beta barium methaborate (β-BaB₂O₄), lithiumtriborate (LiB₃O₅), cesium triborate (CsB₃O₅) and other borate crystalsare known. These frequency converting nonlinear optical crystals forgenerating blue and ultraviolet rays will pass wavelengths of 200 nm andbelow, and have a large nonlinear optical coefficient.

[0005] It is very difficult, however, to grow crystals of β-BaB₂O₄, oneof such frequency converting nonlinear optical crystals, because of thetendency of causing phase transition in the production process. Further,angular allowance is very tight, and thus this particular substance hasa very low level of generality.

[0006] Furthermore, for LiB₃O₅ other frequency converting nonlinearoptical crystal, the growth timeis very long as a result of flux growthin the production process, and this crystal is only good for phasematching for rays down to about 555 nm wavelengths for second harmonicgeneration. This crystal is used, for example, for generation of thethird harmonic (wavelength: 355 nm) of Nd-YAG lasers, but cannot be usedfor generating the fourth harmonic (wavelength: 266 nm).

SUMMARY OF THE INVENTION

[0007] The present invention was developed to overcome theabove-mentioned drawbacks of the prior art and has an object to providea cesium-lithium-borate crystal and crystals with substituted chemicalcompositions which are high-performance frequency converting nonlinearoptical crystals that pass shorter wavelengts, have a high conversionefficiency, and have a large temperature and angular allowance, a methodfor manufacturing such crystals, and a method for utilizing same.

[0008] As means to solve the above-mentioned problems, the presentinvention provides a cesium-lithium-borate crystal having a chemicalcomposition expressed as CsLiB₆O₁₀.

[0009] The present invention provides also a substitutedcesium-lithium-borate crystal having a chemical composition expressed bythe following formula:

Cs_(1−x)Li_(1−y)M_(x+y)B₆O₁₀

[0010] (where, M is at least one alkali metal element other than Cs andLi, and x and y satisfy the relationship of 0≦x≦1 and 0≦y≦1, and x and ynever take simultaneously a value of 0 or 1), or the following formula:

Cs_(2(1−z))Li₂L_(z)B₁₂O₂₀

[0011] (where, L is at least one alkali earth metal element, and 0z<1).

[0012] Furthermore, the present invention provides also a method formanufacturing the above-mentioned crystals by heating and melting a rawmaterial mixture of constituent elements, a method for manufacturing theabove-mentioned crystals through growth by the melt methods comprisingthe crystal pulling method and top seeded kyropoulos method, and amethod for manufacturing the above-mentioned crystals through growth bythe flux method.

[0013] The present invention furthermore provides a frequency converterand an optical parametric oscillator provided with the abovecesium-lithium-borate crystal or the crystal with a substituted chemicalcomposition as optical means.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 shows a structural view illustrating an example ofstructure of a five-zone furnace for growing a cesium-lithium-boratecrystal which is an embodiment of the present invention.

[0015]FIG. 2 shows a three-dimensional structural diagram illustratingthe structure of the cesium-lithium-borate crystal of the presentinvention.

[0016]FIG. 3 shows a graph illustrating transmission spectrum of thecesium-lithium-borate crystal of the present invention.

[0017]FIG. 4 shows a graph illustrating the refractive index dispersioncurve for the cesium-lithium-borate crystal of the present invention.

[0018]FIG. 5 shows a relational diagram illustrating the relationshipbetween the mixing ratio of boron oxide (B₂O₃) and temperature duringmanufacture of the cesium-lithium-borate crystal of the presentinvention.

[0019]FIG. 6 shows a graph illustrating the relationship between thephase matching angle θ and the incident laser wavelength of a crystalcapable of generating the second harmonic (SHG) in Nd:YAG laser with thecesium-lithium-borate crystal which is an embodiment of the presentinvention.

[0020]FIG. 7 shows a graph illustrating the relationship between thewalk-off angle and the incident laser wavelength in Nd:YAG laser of acesium-lithium-borate crystal which is an embodiment of the presentinvention.

[0021]FIG. 8 shows a graph illustrating the fourth harmonic generatingcharacteristics of a cesium-lithium-borate crystal which is anembodiment of the present invention.

[0022]FIG. 9 shows a graph illustrating a theoretical curve ofnon-critical phase matching wavelength resulting from generation of asum frequency, and wavelength of the resultant sum frequency for acesium-lithium-borate crystal which is an embodiment of the presentinvention.

[0023]FIG. 10 is a photograph taking the place of a drawing of the fifthharmonic, and the second and fourth harmonics of Nd:YAG laser availableby the generation of sum frequency for a cesium-lithium-borate crystalwhich is an embodiment of the present invention.

[0024]FIG. 11 shows a schematic representation of the beam pattern ofthe fifth harmonic, and the second and fourth harmonics of Nd:YAG laseravailable by the generation of sum frequency for a cesium-lithium-boratecrystal which is an embodiment of the present invention.

[0025]FIG. 12 shows a graph illustrating the phase matching tuning curveof Type-I OPO using a cesium-lithium-borate crystal which is anembodiment of the present invention with wavelengths of the excitedlight of 213 nm, 266 nm, 355 nm and 532 nm.

[0026]FIG. 13 shows a A graph illustrating the phase matching tuningcurve of Type-II OPO using a cesium-lithium-borate crystal which is anembodiment of-the present invention with a wavelength of the excitedlight of 532 nm; and

[0027]FIG. 14 shows a graph illustrating power X-ray diffraction dataregarding Cs_(1−x)LiRb_(x)B₆O₁₀ crystal.

DETAILED DESCRIPTOIN OF THE INVENTION

[0028] The present inventor noticed the fact that the borate crystalssuch as beta-barium methaborate (β-BaB₂O₄), lithium triborate (LiB₃O₅),and cesium triborate (CsB₃O₅), which are conventionally used asfrequency converting nonlinear optical crystals for generating blue andultraviolet rays, were generally borate crystals containing independentmetals, and found that high-performance borate crystals never seenbefore can be realized by adding a plurality of kinds of metal ions.

[0029] The present inventor produced several kinds of borate crystalscontaining ions of two or more kinds of metals such as an alkali metaland an alkali earth metal, and irradiated them with an Nd YAG laser(wavelength: 1,064 nm) to generate the second harmonic (wavelength: 532nm) in order to find an optimum metal combination through a number ofexperimental verifications.

[0030] As a result, he has found that the borate crystals containingboth Cs and Li, in particular, generate very strong second harmonic, andhas developed totally new crystals including the cesium-lithium-boratecrystal and crystals with substituted chemical compositions thereof ofthe present invention.

[0031] Prescribed crystals of the present invention are manufactured byheating and melting a mixture of raw materials such as cesium carbonate(Cs₂CO₃), lithium carbonate (Li₂CO₃) and boron oxide (B₂O₃). Thereaction is expressed, for example, by the following formula:

Cs₂CO₃+Li₂CO₃+6B₂O₃−2CsLiB₆O₁₀+2CO₂

or

Cs₂CO₃+Li₂CO₃+12H₃BO₃−2CsLiB₆O₁₀+2CO₂→+18H₂O→

[0032] As a cesium-lithium-borate crystal substituted by an alkali metalelement or an alkali earth metal element (M) other than Cs and Li, thereis conceivable a crystal using an arbitrary alkali metal element otherthan Cs and Li, as expressed by the following formula:

Cs_(1−x)Li_(1−y)M_(x+y)B₆O₁₀

[0033] Examples of composition include a composition with 0<x≦0.01 whenthe alkali metal element (M) is Na (sodium), a composition with 0<x≦0.1when M is K (potassium), and a composition with 0<x≦1 when M is Rb(rubidium) as being within a suitable range from the point of view ofmanufacture and physical properties. It is needless to mention that aplurality of alkali metal elements may be added.

[0034] By adding these alkali metal ions, it is possible to change therefractive index and phase matching angle, and to improve angularallowance and temperature allowance, and by simultaneously causing astructural change in crystal, a more stable crystal which is hard tocrack and free from becoming white-muddy is available.

[0035] In the case of the following formula:

Cs_(2(1−z))Li₂L_(z)B₁₂O₂₀

[0036] Ions of alkali earth metals (L) such as Ba, Sr, Ca, Mg and Be areadded. It is needless to mention that a plurality of alkaliearth metalelements may be added.

[0037] As in the case with alkali metals alone, addition of these alkaliearth metal ions permits changing the refractive index and the phasematching angle, and improvement of angular allowance and temperatureallowance. At the same time, it is possible to obtain a more stablecrystal by changing crystal structure.

[0038] In the present invention, the above-mentioned crystals can beused in frequency wave conversion or optical parametric oscillation(OPO). In other words, the present invention permits achievement of anoptical apparatus provided with the above-mentioned crystals.

[0039] Now, the present invention will be described in further detail bymeans of examples. It is needless to mention that the present inventionis not limited to the following examples.

EXAMPLE 1

[0040] Raw materials used were Cs₂CO₃, Li₂CO₃, and B₂O₃. These materialswere mixed in a mol ratio of 1:1:6, and heated and melted to synthesizecrystals. The melting point of the crystal was 848° C.

[0041] There was obtained a transparent crystal measuring 30×25×25 mmafter approximately two weeks' growth period using the top seed methodand temperature decrease method to grow the crystal from the melt.

[0042] The chemical structural formula of the crystal obtained isCsLiB₆O₁₀ according to the result of compsitional analysis such as ICPemission spectral analysis and ICP mass spectroscopy. Measurement of themelting point through a differential thermal analysis revealed a meltingpoint of 848° C. of this cesium-lithium-borate crystal. This crystalbelongs to tetragonal system (space group 142d) according to the resultof an X-ray structural analysis. Further, this cesium-lithium-boratecrystal is transparent for the visible radiations, and it passes lightof wavelengths down to 180 nm.

[0043] The effective second-order nonlinear optical constant wasd_(eff)=4d_(KDP) according to the result of analysis by the powdermethod.

[0044] Further, a part of this cesium-lithium-borate crystal measuring30×25×25 mm was cut by the angle of phase matching, and polished. Thepolished crystal was then irradiated by 1.06 μm neodymium YAG laserlight. Green rays of 0.53 μm wavelength (the second harmonic), wasobtained efficiently.

[0045] A cesium-lithium-borate crystal measuring 10 diameter ×20 mm wasgrown by the revolving pull-up method (rate of revolution: 10 rpm;pull-up rate: 0.5 mm/h). This was confirmed to be the same as thatdescribed above.

EXAMPLE 2

[0046] A cesium-lithium-borate crystal (CsLiB₆O₁₀) comprising astoichiometric chemical composition was manufactured by heating andmelting a mixture of cesium carbonate (Cs₂CO₃), lithium carbonate(Li₂CO₃) and boron oxide (B₂O₃), and the resultant cesium-lithium-boratecrystal was grown by the top seeded kyropoulos method in a five-zonefurnace. FIG. 1 shows the structure of the five-layer controlled growingfurnace used for growing the crystal. This five-layer controlled growingfurnace has a structure in which a cylindrical platinum crucible isinstalled in an upright five-stage resistance heating furnace capable ofkeeping a uniform temperature in the furnace. The seed crystal ofCsLiB₆O₁₀ was attached to a platinum rod. The growing crystal wasrotated at a rate of 15 rpm with inverting the rotation direction inevery three minutes. Temperature in the platinum crucible was kept at848° C., the melting point of the crystal. This permitted growth of atransparent and high-quality cesium-lithium-borate crystal, measuring2.9 cm×2.0 cm×2.2 cm, free from cracks in about four days. Thisrepresents a very short period of growth as compared with theconventional growth of a nonlinear optical borate crystal for frequencyconversion. It is thus possible to easily grow a cesium-lithium-boratecrystal in a very short period of time for growth by the application ofthe growing method of the cesium-lithium-borate crystal of the presentinvention.

[0047] According to the result of crystal structural analysis by meansof a Rigaku AFC5R X-ray diffraction apparatus, the cesium-lithium-boratecrystal was a tetragonal crystal belonging to the space group 142dsymmetrical group, with a crystal lattice constant of a=10,494 andc=8,939 Å, and a calculated density of 2.461 g/cm. FIG. 2 shows thethree-dimensional structure of this cesium-lithium-borate crystal, whichsuggests a structure in which a cesium atom is present in a channel ofborate ring comprising boron and oxygen. It is clear that this crystalhas a structure quite different from that of LiB₃O₅ or CsB₃O₅ (both areor the rhombic), nonlinear optical crystal so far commonly used.

[0048] This cesium-lithium-borate crystal was transparent relative to alight having a wavelength of from 180 nm to 2750 nm, according to theresult of measurement of transmission spectra. FIG. 3 shows thetransmission spectrum in the short wavelength region. As is clear fromFIG. 3, the crystal had an absorption edge of 180 nm which was shorterby about 9 nm than that of the conventional β-B_(a)B₂O₄ (189 nm).

[0049] The refractive index of this cesium-lithium-borate crystal wasmeasured by the prism method within a wavelength range of from 240 nm to1,064 nm. FIG. 4 shows a dispersion curve of the refractive index. InFIG. 4, “n_(o)” represents the refractive index for the normal light,and “n_(e)” indicates that for an abnormal light.

[0050] The approximation formula of refractive index (Sellmeier'sequation) as available from this refractive index dispersion curve is asfollows: $\begin{matrix}{n_{o}^{2} = {2.19974 + \frac{1.18388 \times 10^{- 2}}{\lambda^{2} - {8.77048 \times 10^{- 3}}} - {8.52469 \times 10^{- 5}\lambda^{2}}}} \\{n_{e}^{2} = {2.05386 + \frac{9.4403 \times 10^{- 3}}{\lambda^{2} - {8.62428 \times 10^{- 3}}} - {7.82600 \times 10^{- 5}\lambda^{2}}}}\end{matrix}$

EXAMPLE 3

[0051] The relationship between the mixing ratio of B₂O₃ and temperatureupon manufacture of this cesium-lithium-borate crystal was determined.The mixing ratio of B₂O₃ was varied within a range of from 66.7% to83.3% while keeping a constant mixing ratio of 1:1 between the initialraw materials Cs₂CO₃ and Li₂CO₃, and the melting point of the crystalwas determined by placing a sintered powder of the resultant mixtureinto a differential thermal analyzer. FIG. 5 is a graph illustrating therelationship between the mixing ratio of B₂O₃ and temperature in thiscase. In FIG. 5, Cs₂CO₃ and Li₂CO₃ mixed at a ratio of 1:1 arerepresented by Cs₂O Li₂O. As is evident from FIG. 5, a CsLiB₆O₁₀ crystalwas stably available with a mixing ratio of B₂O₃ within a range of from66.7% to 81.8%. With a mixing ratio of B₂O₃ of under 66.7%, CBOprecipitated along with CsLiB₆O₁₀ crystal, and with a mixing ratiowithin a range of from 81.8 to 83.8%, unknown crystals other thanCsLiB₆O₁₀ crystal precipitated simultaneously, thus resulting inunstable manufacture of the crystal. In the manufacture of thiscesium-lithium-borate crystal, therefore, the mixing ratio of B₂O₃, aninitial raw material, should preferably be kept within a range of from66.7% to 81.8%. The CsLiB6O10 crystal stably manufactured can meltcongruently at 848° C.

[0052] Since this cesium-lithium-borate crystal is a congruently meltingcrystal, it is possible to grow a high-quality crystal with a constantcomposition easily within a short period of time through adoption of theTop-seeded kyropoulos method, crystal pulling method or the flux method,as compared with the conventional β-BaB₂O₄ tending to easily cause phasetransition during growth from the melt and LiB₃O₅ requiring a longgrowth period for flux growth.

EXAMPLE 4

[0053] A cesium-lithium-borate crystal was manufactured by heating andmelting 12 kg mixture of cesium carbonate (Cs₂CO₃), lithium carbonate(Li₂CO₃) and boron oxide (B₂O₃) mixed at a ratio of 1:1:5.5 (B₂O₃accounting for 73.3%). The resultant cesium-lithium-borate crystal waslargely grown by the flux method in a five-zone furnace. As largecrystal growth requires a large temperature drops the flux method issuitable. For the purpose of growing a large crystal, a large platinumcrucible having a diameter of 20 cm and a height of 20 cm was employed.In this Example, the growth saturation temperature was measured to be845° C. The crystal was grown by reducing the growing temperature from845° C. to 843.5° C. at a daily rate of about 0.1° C. A largetransparent crystal measuring 13 cm×12 cm×10 cm and weighing about 1.6kg could be grown in about 12 days. In this crystal growth, there was nounstable growth such as hopper growth observed in the conventionalgrowth of LiB₃O₅ crystal, proving a very stable growth.

EXAMPLE 5

[0054] By using the cesium-lithium-borate crystal (CsLiB₆O₁₀) of thepresent invention as a frequency converting nonlinear optical crystal ofan Nd:YAG laser, the second harmonic (SHG: wavelength: 532 nm) of theNd:YAG laser (wavelength: 1,064 nm) was generated.

[0055]FIG. 6 illustrates the relationship between the phase matchingangle θ of the crystal permitting second harmonic generation (SHG) andthe input laser wavelength. In FIG. 6, the dotted line represents thecalculated values based on Sellmeier's equation for Type-I SHG, thesolid line, the calculated values for Type-II SHG by Sellmeier'sequation, and the black plots are observed values. The limit of SHGwavelength is 477 nm in Type-I, and 640 nm in Type-II. As is clear fromFIG. 6, for example, the Type-I SHG of Nd:TAG laser beam having awavelength of 1,064 nm shows an incident angle of 29.6° in calculationand about 30° in observation. The Type-I SHG of Nd:YAG laser beam havinga wavelength of 532 nm gives an incident angle of 62.5° in calculationand 62° in observation. There is suggested a satisfactory agreement.

[0056]FIG. 7 is a graph illustrating the relationship between thewalk-off angle and the wavelength for Type-I SHG obtained throughcalculation from Sellmeier's equation. In FIG. 7, the solid linerepresents CsLiB₆O₁₀ of the present invention, and the dotted line, theconventional β-BaB₂O₄.

[0057] Table 1 shows, for CsLiB₆O₁₀ of the present invention and theconventional β-BaB₂O₄ Type-I SHG, with an incident wavelengths of 1,0649μm and 532 nm, calculated values of phase matching angle θ, effectivenonlinear optical coefficient d_(eff), angular allowance Δθ.L, spectralallowance Δλ.L, temperature allowance ΔT. L, walk-off angle, and laserdamage threshold value. As to the refractive index for β-BaB₂O₄necessary for calculating these values, those in literature released in“J. Appl. Phys. Vol. 62, D. Eimerl, L, Davis, S. Velsko, B. K. Grahamand A. Zalkin (1987), p. 1968.”

[0058] The effective nonlinear optical constant deff was derived fromcomparison with SHG of KH₂PO₄ (KDP) crystal. CsLiB₆O₁₀ has a crystalstructure identical with that of KDP.

[0059] The second-order nonlinear optical coefficient is expressed asd₃₆, where d₃₆(CLBO)=2.2×d₃₆(KDP)=0.95 pm/r, and its relationship withd_(eff) is d_(eff) =−d₃₆ sin θ sin 2φ. The value of d_(eff) wascalculated by means of this formula. A value of 0.435 pm/V was used asthe standard value of d₃₆ of KDP.

[0060] The angular allowance Δθ.L and the spectral allowance Δλ. L werecalculated in accordance with Sellmeier's equation.

[0061] The temperature allowance ΔT. L, which could not be obtained fromcalculation, was actually measured within a range of from 20° C. to 150°C. TABLE 1 Fundamental Phase- Walk-off Damage wavelength matchingd_(eff) Δθl Δλl ΔTl angle threshold (nm) Crystal angle (θ) (pm/V) (mrad· cm) (nm · cm) (° C. · cm) (deg) (GW/cm²) 1064 CLBO 29.6 0.47 1.02 7.031.78 26 BBO 21 2.06 0.51 2.11 3.20 13.5 532 CLBO 62.5 1.01 0.49 0.13 9.41.83 BBO 48 1.85 0.17 0.07 4.5 4.80 488 CLBO 77.9 1.16 0.84 0.09 0.98BBO 1.88 0.16 0.05 4.66

[0062] As is clear from Table 1, the CsLiB₆O₁₀ of the present inventionhas a smaller effective nonlinear optical constant as compared with theconventional BBO. The CsLiB₆O₁₀ of the invention is however larger inangular allowance, wavelength allowance and temperature allowance andsmaller in walk-off angle. The cesium-lithium-borate crystal of thepresent invention therefore permits more effective frequency conversionthan that with the conventional nonlinear optical crystal.

EXAMPLE 6

[0063] The fourth harmonic (4HG; wavelength: 266 nm) of Nd:YAG laser(wavelength: 1,064 nm) was generated by using the cesium-lithium-boratecrystal (CsLiB₆O₁₀) of the present invention in Nd:YAG laser. The secondharmonic (SHG) of Q-switch laser having a pulse width of 8 nanosecondswas employed as the incident light with a beam diameter of 4 mm andrepetition rate at 10 Hz. FIG. 8 is a graph illustrating therelationship between the energy output of the incident light SHG and theenergy output of 4HG, i.e., generation characteristics of the fourthharmonic. In FIG. 8, the solid line represents the CsLiB₆O₁₀ of thepresent invention, and the dotted line indicates β-BaB₂O₄. The samplelength was 9 mm for CsLiB₆O₁₀ and 7 mm for β-BaB₂O₄. As is evident fromFIG. 8, according as the energy of the incident light SHG becomeslarger, β-BaB₂O₄'s 4HG energy shows a tendency toward saturation,whereas CsLiB₆O₁₀ of the invention is proportional to the square of theincident energy, and in the high incident energy region with a highenergy of incident light, a 4HG output energy larger than that ofβ-BaB₂O₄ is available. The cesium-lithium-borate crystal of the presentinvention can therefore be used as a very excellent wavelengthconverting nonlinear optical crystal capable of generating ultravioletrays of a high output energy.

EXAMPLE 7

[0064] The fifth harmonic (5HG; wavelength: 213 nm) of Nd:YAG laser(wavelength: 1,064 nm) was generated by using the cesium-lithium-boratecrystal (CsLiB₆O₁₀) of the present invention in the Nd:YAG laser.

[0065]FIG. 9 is a graph illustrating the results of calculation of afrequency capable of generating a sum frequency (107 ₁+ω₂=ω₃) of twofrequencies (ω₁ and ω₂) in CsLiB₆O₁₀ of the invention, as derived fromSellmeir's equation. The abscissa represents the light wavelength λ₁corresponding to the frequency ω₁, and the ordinate represents the lightwavelength λ₂ corresponding to the frequency ω₂ and the light wavelengthλ₃ corresponding to the frequency ω₃. In FIG. 9, the region shadowedwith oblique lines above the solid line is the region in which a sumfrequency can be generated. The dotted line represents the wavelength λ₃available as a result of a sum frequency. For example, assuming that anNd:YAG laser has a basic wave (wavelength: 1,064 nm) ω, then ω+4ω=5ω ispossible. In other words, the fifth harmonic can be generated by the sumfrequency of the basic wave and the fourth harmonic. Generation of thefifth harmonic by adding the second and third harmonics is howeverimpossible.

[0066] In FIG. 9, the black plots represent the wavelength availablefrom sum frequency of ω+4ω and that available from sum frequency of2ω+3ω. Presence of only black plots representing ω+4ω in the slashedregion reveals that the fifth harmonic can be generated only from thesum frequency of ω+4ω. As is evident from the dotted line (wavelength:λ₃) in FIG. 9, a wavelength of even under 200 nm can be generated from asum frequency by properly selecting wavelengths λ₁ and λ₂.

[0067]FIG. 10 illustrates a photograph of the beam pattern of the fifthharmonic of Nd:YAG laser generated with CsLiB₆O₁₀ of the presentinvention. FIG. 11 is a schematic view of crystal arrangement upongeneration of the second harmonic, the fourth harmonic and the fifthharmonic in this case. Table 2 shows energy values of the individualfrequencies of CsLiB₆O₁₀(LLBO) of the invention and the conventionalβ-BaB₂O₄(BBO). As is clear from Table 2, while 5ω available from theconventional β-BaB₂O₄(BB0) is 20 mJ, what is available from CsLiB₆O₁₀ ofthe invention is a higher output of 35 mJ. The cesium-lithium-boratecrystal of the present invention can therefore generate the fifthharmonic of a higher output than the conventional β-BaB₂O₄, and can beused as a nonlinear optical crystal for generating a very excellentfifth harmonic. The beam pattern obtained from FIG. 10 is closest tocircle, suggesting that it is possible to generate second harmonicsthroughout the entire beam. This is attributable to a larger angularallowance Δθ.L and temperature allowance ΔT. L of CLBO than BBO. TABLE 2Harmonic 2ω 4ω 5ω Crystal Pouwer (mJ) 350 110 35 CLBO Power (mJ) 500  8020 BBO

EXAMPLE 8

[0068] The output of wavelength of 488 nm of Ar laser was converted intoa second harmonic by means of the cesium-lithium-borate crystal of thepresent invention. Table 1 presented above shows, for the CsLiB₆O₁₀ ofthe invention and the conventional β-BaB₂O₄ relative to Type-I SHG withan incident wavelength of 488 nm, calculated values of phase matchingangle θ, effective nonlinear optical coefficien d_(eff), angularallowance. Δθ.L, spectral allowance Δλ. L, and walk-off angle. The samecalculating methods as in the Example 5 were used for thesecalculations. As is clear from Table 1, CsLiB₆O₁₀ has a walk-off angleof 0.98° which is very close to the noncritical phasematching. Thisdemonstrates that the cesium-lithium-borate crystal of the inventiongives a very high conversion efficiency as compared with theconventional β-BaB₂O₄.

EXAMPLE 9

[0069] The cesium-lithium-borate crystal of the present invention wasused for optical parametric oscillation (OPO).

[0070] Optical parametric oscillation (OPO) is a process of wavelengthconversion comprising exciting a nonlinear polarization within anonlinear optical crystal with a laser beam, thereby dividing the energyof the excited beam through nonlinear oscillation of polarized electronsinto a signal light and an idler light. Because of the possibility totune a wavelength region within a wide range, a wider application of OPOis expected. The cesium-lithium-borate crystal of the invention, havinga relatively large effective nonlinear optical coefficient, can supply alonger crystal length, because of the lasiness of growing a largecrystal. and further to a larger power density of the excited beambecause of the high laser damage threshold value, thus providingexcellent characteristics as an OPO crystal.

[0071]FIG. 12 is a phase matching tuning curve diagram illustrating therelationship between the wavelengths of the signal light produced inType-I with excited light wavelengths of 213 nm, 266 nm, 355 nm and 532nm, and the corresponding phase matching angles. FIG. 13 is a phasematching tuning curve diagram illustrating the relationship between thewavelength of the signal light produced in Type-II with a wavelength ofthe excited light of 532 nm and the corresponding phase matching angle.As is clear from FIGS. 12 and 13, the cesium-lithium-borate crystal ofthe present invention exhibits excellent properties also as an OPOcrystal.

[0072] Particularly OPO based on excitation with the fourth harmonic(wavelength: 266 nm) of Nd:YAG laser gives a variable-wavelength laserbeam near 300 nm. This has been impossible in the conventional β-BaB₂O₄because of the small angular allowance Δθ.L and the large walk-off.

EXAMPLE 10

[0073] An Rb (rubidium)-substituted cesium-lithium-borate crystalCs_(1−x)LiRb_(x)B₆O₁₀ was manufactured in the same manner as in theExample 2.

[0074] According to the results of evaluation of the resultant crystalby the powder X-ray diffraction method, as shown in FIG. 14,particularly, the interval between reflection peak of (312) plane andreflection peak of (213) plane becomes gradually arrower by sequentiallyincreasing the amount of added Rb from x=0.2, to 0.5 and then 0.7 to theX-ray diffraction pattern of the sample (Rb, x=O) not added with Rb.This shows that Cs and Rb enter into the crystal at an arbitrary ratio.The crystal added with Rb arbitrarily is the same tetragonal crystal asCLBO not added with Rb, and the lattice constant varies accordingly.

[0075] Since it is possible to add Rb ions in an arbitrary amount, it ispossible to change the refractive index of the crystal, and this revealsthe possibility to improve the phase matching angle, angular allowanceand temperature allowance.

[0076] Similarly, crystals with amounts (x) of added Rb of under 0.1were manufactured. It was confirmed as a result that the stability ofcrystal structure was more satisfactory.

EXAMPLE 11

[0077] Crystals were manufactured in the same manner as in the Example10 except that K or Na was added in place of Rb. Availability ofhigh-quality crystals was confirmed with a constituent ratio (x) ofunder 0.1 for κ (potassium), and under 0.01 for Na (sodium).

[0078] Crystals in which K or Na was coexistent with Rb were alsomanufactured. In this case, in a composition:

Cs₁−xLi₁−yRb_(x)(Na, K)_(y)B₆O₁₀,

[0079] a more stable crystal was obtained with 0<x≦1 and 0<y <0.1.

EXAMPLE 12

[0080] Crystals were manufactured in the same manner as in the Example10 except that an alkali earth metal element was added in place of analkali metal.

[0081] For example, in the case of a compositionCs_(2(1-Z))Li₂Ba_(Z)B₁₂O₂₀, it was confirmed that a stable crystal isavailable with 0<z-≦0.1.

[0082] According to the present invention, as described above in detail,a novel cesium-lithium-borate crystal and substitutedcesium-lithium-borate crystals are provided. These crystals permitconversion of frequency, has a high converting efficiency, and widetemperature allowance and angular allowance, and can be used as ahigh-performance frequency converting crystal. Furthermore, theCsLiB₆O₁₀ crystal has a low melting point of 848° C., and because of thecongruency of CsLiB₆O₁₀ crystal, it is possible to easily grow a largehigh-quality crystal having a stable composition by the application ofthe melt methods based on the Top-seeded kilopoulos method, crystalpulling method or the flux method.

What is claimed is:
 1. A cesium-lithium-borate crystal having a chemicalcomposition expressed as CsLiB₆O₁₀.
 2. A substitutedcesium-lithium-borate crystal having a chemical composition expressed bythe following formula: Cs_(1−x)Li_(1−y)M_(x+y)B₆O₁₀ (where, M is atleast one alkali metal element other than Cs and Li, and x and y satisfythe relationship of 0≦×≦1 and 0≦y≦1, and x and y never takesimultaneously a value of 0 or 1).
 3. A substitutedcesium-lithium-borate crystal having a chemical composition expressed bythe following formula: Cs_(2(1−z))Li₂L_(z)B₁₂O₂₀ (where, L is at leastone alkali earth metal element, and 0<z<1).
 4. A crystal as claimed inclaim 1, at least one alkali metal ion other than Cs and Li is added forimproving of optical and mechanical properties.
 5. A crystal as claimedin 1, at least one alkali earth metal ion is added for improving ofoptical and mechanical properties.
 6. A method for manufacturing acrystal as claimed in any one of claims 1 to 5, which comprises the stepof heating and melting a raw material mixture of constituent elements,thereby manufacturing said crystal.
 7. A method for manufacturing thecrystal as claimed in any one of claims 1 to 5, which comprises the stepof growing the crystal by the melt method.
 8. A method for manufacturingthe crystal as claimed in any one of claims 1 to 5, which comprises thestep of growing the crystal by the flux method.
 9. A frequencyconverting apparatus which is provided with any of the crystals asclaimed in claims 1 to 5 as optical means.
 10. An apparatus as claimedin claim 9 for generating the second, third, fourth or fifth harmonic ofa laser.
 11. An optical parametric oscillator which is provided with anyof the crystals as claimed in claims 1 to 5 as optical means.